Photovoltaic device

ABSTRACT

A photovoltaic device including a plurality of unit devices stacked, each unit device comprising a silicon-based non-single-crystal semiconductor material and having a pn or pin structure, in which an oxygen atom concentration and/or a carbon atom concentration has a maximum peak in the vicinity of a p/n interface between the plurality of unit devices.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a photovoltaic device comprising asilicon-based non-single-crystal semiconductor material.

2. Related Background Art

A photovoltaic device is a semiconductor device for converting opticalenergy, such as solar light, into electrical energy. As a semiconductormaterial for such a device, an amorphous material represented byamorphous silicon (a-Si:H) is attracting attention, and is beingactively investigated since it is inexpensive, enables a large areaformation and a thin film formation, and has a large freedom in thecomposition, thereby allowing for control of electrical and opticalcharacteristics over a wide range.

In a photovoltaic device comprising the above-mentioned amorphousmaterial, particularly an amorphous solar cell, an improvement in aphotoelectric conversion efficiency is an important object.

In order to attain such an object, U.S. Pat. No. 2,949,498 discloses theuse of so-called tandem cells, formed by stacking a plurality of solarcells each having a unit device structure. Such tandem cells improve theconversion efficiency by stacking devices of different band gaps andthereby efficiently absorbing different portions of the spectrum ofsolar light. Tandem cells are so designed that, in comparison with aband gap of a device positioned at a light entrance side (the so-calledtop layer among the stacked devices), the so-called bottom layer,positioned under such top layer, has a narrower band gap. There is alsobeing investigated a three-layer tandem cell (“triple cell”) having amiddle layer between the top layer and the bottom layer.

Also in order to facilitate collection of holes having a shorterdiffusion distance among electron-hole pairs generated from an incidentlight, there is often adopted a configuration in which a p-type layer ispositioned at a side of a transparent electrode, namely at a lightentering side, thereby increasing an overall light collectingefficiency, and a substantially intrinsic semiconductor (“i-type layer”)is provided between a p-type layer and an n-type layer.

Also, an improvement in a short-circuit current (Jsc) is achieved byemploying a microcrystalline silicon in the p-type layer at the lightentrance side, utilizing the properties of the microcrystalline siliconhaving a high conductivity and a small absorption coefficient in a shortwavelength region. Also, the microcrystalline silicon, having a widerband gap than in the amorphous silicon, shows a higher efficiency forimpurity doping and provides a larger internal electric field in thephotovoltaic device. As a result, there are reported improvements in anopen-circuit voltage (Voc) and in the photoelectric conversionefficiency (“Enhancement of open circuit voltage in high efficiencyamorphous silicon alloy solar cells”, S. Guha, J. Yang, P. Nath and M.Hack: Appl. Phys. Lett., 49 (1986) 218).

However, in such photovoltaic devices, it is difficult to stably controlinterfacial characteristics of a junction between the p-type layer andthe n-type layer, and a change in a junction state or in an amount ofimpurity causes an increase in a serial resistance and an associateddeterioration of I-V (current-voltage) characteristics, thus leading toa fluctuation in the characteristics.

SUMMARY OF THE INVENTION

In consideration of the foregoing, an object of the present invention isto provide a photovoltaic device capable of stabilizing a structure of ap/n interface and improving interfacial characteristics and filmadhesion, thereby achieving a high photoelectric conversion efficiency.

The present invention provides a photovoltaic device comprising aplurality of unit devices stacked, each unit device comprising asilicon-based non-single-crystal semiconductor material and having a pnor pin structure, in which an oxygen atom concentration has a maximumpeak in a vicinity of a p/n interface between the plurality of unitdevices. In such a photovoltaic device, the concentration at the peak ofthe oxygen atom concentration (peak oxygen concentration) is preferably“1×10¹⁸ atoms/cm³ or higher and 1×10²² atoms/cm³ or lower”.

Also, the present invention provides a photovoltaic device comprising aplurality of unit devices stacked, each unit device comprising asilicon-based non-single-crystal semiconductor material and having a pnor pin structure, in which a carbon atom concentration has a maximumpeak in a vicinity of a p/n interface between the plurality of unitdevices. In such a photovoltaic device, the concentration at the peak ofthe carbon atom concentration (peak carbon concentration) is preferably“10¹⁶ atoms/cm³ or higher and 10²⁰ atoms/cm³ or lower”.

Also, the present invention provides a photovoltaic device comprising aplurality of unit devices stacked, each unit device comprising asilicon-based non-single-crystal semiconductor material and having a pnor pin structure, in which an oxygen atom concentration and a carbonatom concentration have a maximum peak in a vicinity of a p/n interfacebetween the plurality of unit devices. In such a photovoltaic device,the concentration at the peak of the oxygen atom concentration (peakoxygen concentration) is preferably 1×10¹⁸ atoms/cm³ or higher and1×10²² atoms/cm³ or lower, and the concentration at the peak of thecarbon atom concentration (peak carbon concentration) is preferably“10¹⁶ atoms/cm³ or higher and 10²⁰ atoms/cm³ or lower”.

In such photovoltaic devices provided by the present invention, a p-typelayer at the aforementioned p/n interface is preferably formed fromhydrogenated microcrystalline silicon, and a p-type layer in theaforementioned unit device is preferably positioned at the lightentrance side.

In such photovoltaic device, by controlling content of oxygen and/orcarbon in the vicinity of the p/n interface, it is possible to stabilizethe structure of the aforementioned interface, thereby improving theinterfacial characteristics and the film adhesion.

In the present invention, the “p/n interface” in the photovoltaic deviceformed by stacking a plurality of unit devices each comprising asilicon-based non-single-crystal semiconductor material and having a pnor pin structure means a stacking interface between the stacked unitdevices, in other words, between the p-type semiconductor layer of oneunit device and the n-type semiconductor layer of another unit deviceadjacent to the one unit device. In the following, even when notparticularly specified, the term “p/n interface” in the presentspecification means the above-described interface between the unitdevices, and it does not mean a p/n interface in an ordinary p/njunction semiconductor device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic, sectional view showing a configuration of aphotovoltaic device of the present invention;

FIG. 2 is a graph showing an oxygen concentration distribution in thefilm thickness direction of a triple cell prepared in Examples;

FIG. 3 is a graph showing a relationship between a peak oxygenconcentration in a top n-type layer and an initial conversionefficiency, in a triple cell prepared in Examples;

FIG. 4 is a graph showing a carbon concentration distribution in thefilm thickness direction of a triple cell prepared in Examples;

FIG. 5 is a graph showing a relationship between a peak carbonconcentration in a middle p-type layer and an initial conversionefficiency, in a triple cell prepared in Examples; and

FIG. 6 is a schematic view showing a film-forming apparatus adapted forpreparing the photovoltaic device of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, the present invention will be described in detail withreference to accompanying drawings, but the present invention is notlimited by such description.

FIG. 1 schematically shows a pin-type amorphous solar cell to which thephotovoltaic device of the present invention can be advantageouslyapplied. FIG. 1 shows a solar cell having a structure in which a lightenters from the upper side of the solar cell. In FIG. 1, a main body 100of the solar cell, a bottom layer 114, a middle layer 115, a top layer116, a substrate 101, a lower electrode 102, n-type semiconductor layers103, 106, 109, i-type semiconductor layers 104, 107, 110, p-typesemiconductor layers 105, 108, 111, an upper electrode 112, and acurrent-collecting electrode 113 are shown. The bottom layer 114, themiddle layer 115 and the top layer 116 respectively constitute theaforementioned unit devices.

(Substrate)

A substrate 101, suitable for deposition of semiconductor layers, may beformed by a single crystalline material or a non-single crystallinematerial, and may be electrically conductive or insulating. Also, it maybe translucent or opaque, but preferably has little deformation orstrain and has a desirable level of strength.

Specific examples of the material include a thin plate of a metal suchas Fe, Ni, Cr, Al, Mo, Au, Nb, Ta, V, Ti, Pt, or Pb an alloy thereof,such as brass or stainless steel, or a composite thereof. Other examplesinclude a film or a sheet of heat-resistant synthetic resin, such aspolyester, polyethylene, polycarbonate, cellulose acetate,polypropylene, polyvinyl chloride, polyvinylidene chloride, polystyrene,polyamide, polyimide or epoxy, or a composite thereof with glass fibers,carbon fibers, boron fibers, metal fibers, or the like. Still otherexamples include the above metal thin plate or resinous sheet subjectedto a surface coating of a thin film of a different metal and/or aninsulating thin film, such as of SiO₂, Si₃N₄, Al₂O₃, or AlN bysputtering, evaporation, or plating. Still other examples include glassand ceramics.

In case the substrate is formed by an electrically conductive materialsuch as metal, it may be used directly as a current collectingelectrode. In case it is formed by an electrically insulating materialsuch as a synthetic resin, it is desirable to form in advance, on asurface thereof on which a deposited film is to be formed, a singlemetal or an alloy such as Al, Ag, Pt, Au, Ni, Ti, Mo, W, Fe, V, Cr, Cu,stainless steel, brass, nichrome, SnO₂, In₂O₃, ZnO or ITO, or atransparent conductive oxide (TCO) by plating, evaporation orsputtering. Naturally, even in case the substrate is electricallyconductive such as a metal, a different metal layer or the like may beprovided on a side of the substrate on which the deposited film is to beformed, for example for increasing a reflectivity for a light of alonger wavelength on the substrate surface, or for preventing mutualdiffusion of elements constituting the substrate and the deposited film.

Also, a surface of the substrate maybe a so-called smooth surface or asurface having small irregularities. In case of a surface with smallirregularities, such irregularities may be formed as spherical, conical,or polygonal pyramidal with a maximum height (Rmax) preferably within arange from 50 to 500 nm, whereby the reflection of light on such surfacebecomes random reflection, with an increase in the optical path lengthof the light reflected on such surface. A thickness of the substrate issuitably selected so as to obtain a desired photovoltaic device, but isusually made 10 mm or larger in consideration of a mechanical strengthof the substrate during manufacture and handling.

In the photovoltaic device of the present invention, suitable electrodesare selected according to the configuration of the device. Suchelectrodes generally include a lower electrode, an upper electrode(transparent electrode), and a current collecting electrode (the upperelectrode indicates an electrode provided at a light entrance side,while the lower electrode indicates an electrode provided opposite tothe upper electrode across a semiconductor layer). These electrodes willbe explained in detail in the following.

(Lower Electrode)

A lower electrode 102 employed in the present invention is providedbetween the substrate 101 and an n-type semiconductor layer 103.However, if the substrate 101 is conductive, it can also serve as thelower electrode. However, if the substrate 101 is conductive but has ahigh sheet resistance, the electrode 102 may be provided as alow-resistance electrode for current collecting, or for increasing thereflectance on the substrate surface, thereby achieving efficientutilization of the incident light.

The electrode can be formed by a metal such as Ag, Au, Pt, Ni, Cr, Cu,Al, Ti, Zn, Mo or W or an alloy thereof, and a thin film of such metalis formed by vacuum evaporation, electron beam evaporation orsputtering. It is preferred that the formed metal film does notconstitute a resistance component to an output of the photovoltaicdevice.

Though not illustrated, a transparent conductive layer such as ofconductive zinc oxide may be provided between the lower electrode 102and the n-type semiconductor layer 103. Functionally, the transparentconductive layer can be considered a diffusion preventing layer. Suchdiffusion preventing layer not only prevents a diffusion of a metalelement, constituting the lower electrode 102, into the n-typesemiconductor layer, but is given a certain resistance to preventshortcircuiting between the lower electrode 102 and the upper electrode112 resulting from a defect such as a pinhole in a semiconductor layertherebetween, and serves to generate a multiple interference by a thinfilm thereby enclosing the incident light within the photovoltaicdevice.

(Upper Electrode (Transparent Electrode))

The transparent electrode 112 to be employed in the present inventionpreferably has a light transmittance of 85% or higher in order that thesolar light or the light from a white fluorescent lamp can beefficiently absorbed in the semiconductor layer, and electrically has asheet resistance preferably of 300 Ω/□ or less in order not toconstitute a resistance component to the output of the photovoltaicdevice. Examples of a material having such properties includes anextremely thin semi-transparent film of a metal oxide such as SnO₂,In₂O₃, ZnO, CdO, CdSnO₄, ITO (In₂O₃+Sn₂), a metal such as Au, Al, Cu,etc.

As the transparent electrode 112 deposited on the p-type semiconductorlayer 111 in FIG. 1, there are preferably selected materials withsatisfactory mutual adhesion. These may be prepared by resistance-heatedevaporation, electron beam-heated evaporation, sputtering, spraying,etc., which may be suitably selected according to the purpose.

(Current Collecting Electrode)

A current-collecting electrode to be employed in the present inventionis provided on the transparent electrode 112 for the purpose of reducingthe surface resistance of the transparent electrode 112. A material forthe electrode can be a thin film of a metal such as Ag, Cr, Ni, Al, Au,Ti, Pt, Cu, Mo or W or an alloy thereof. A stack of these thin films mayalso be used in a laminate. Also a shape and an area of the film aresuitably selected so as to secure a sufficient incident light amountinto the semiconductor layer.

For example, the collecting electrode preferably has a shape uniformlyspread in a light-receiving surface of the photovoltaic device, and itsarea preferably is 15% or less of a light-receiving area, morepreferably 10% or less. Also it has a sheet resistance preferably of 50Ω/□ or less, more preferably 10 Ω/□ or less.

(Semiconductor Layer)

Semiconductor layers 103, 104, 105, 106, 107, 108, 109, 110 and 111 areprepared by an ordinary thin film preparing process, and can be preparedby employing a known method such as evaporation, sputtering, highfrequency plasma CVD, microwave plasma CVD, ECR, thermal CVD or LPCVD asdesired. Industrially there is preferably employed a plasma CVD in whicha raw material gas is decomposed by plasma and deposited on thesubstrate.

As a reaction apparatus, there can be employed a batch-type apparatus ora continuous film-forming apparatus as desired. A semiconductor withvalence electron control can also be prepared by simultaneouslydecomposing PH₃ or B₂H₆ gas containing phosphor or boron as aconstituent atom.

(I-Type Semiconductor Layer)

In the present photovoltaic device, as a semiconductor material forforming an advantageously employable i-type semiconductor layer, therecan be employed so-called group IV alloy-based semiconductor materialssuch as a-SiGe:H, a-SiGe:F, or a-SiGe:H:F for producing an i-type layerof amorphous silicon germanium. Also, in the case of forming an i-typesemiconductor layer other than amorphous silicon germanium in tandemcell structure formed by stacking unit devices, there can be employedso-called group IV and group VI alloy-based semiconductor materials suchas a-Si:H, a-Si:F, a-Si:H:F, a-SiC:H, a-SiC:F, a-SiC:H:F, poly-Si:H,poly-Si:F, or poly-Si:H:F, and so-called compound semiconductormaterials, such as those of groups III-V and II-VI.

A raw material gas employed in CVD as a silicon-containing compound canbe a linear or cyclic silane, specifically a gaseous compound or aneasily gasifiable compound such as SiH₄, SiF₄, (SiF₂)₅, (SiF₂)₆,(SiF₂)₄, Si₂F₆, Si₃F₈, SiHF₃, SiH₂F₂, Si₂H₂F₄, Si₂H₃F₃, SiCl₄, (SiCl₂)₅,SiBr₄, (SiBr₂)₅, SiCl₆, SiHCl₃, SiHBr₂, SiH₂Cl₂, or SiCl₃F₃.

Also, a germanium-containing compound can be a linear germane, ahalogenated germanium, a cyclic germane, or an organic germaniumcompound having a halogenated germane, a linear or cyclic germaniumcompound, and an alkyl group, etc., more specifically GeH₄, Ge₂H₆,Ge₃H₈, n-Ge₄H₁₀, t-Ge₄H₁₀, Ge₅H₁₀, GeH₃Cl, GeH₂F₂, Ge(CH₃)₄, Ge(C₂H₅)₄,Ge(C₆H₅)₄, Ge(CH₃)₂F₂, GeF₂, or GeF₄.

(P-type Semiconductor Layer and N-type Semiconductor Layer)

A semiconductor material for constituting the p-type semiconductor layeror the n-type semiconductor layer advantageously employed in the presentphotovoltaic device can be obtained by doping an aforementionedsemiconductor material for constituting the i-type semiconductor layerwith a valence electron controlling agent. For forming such layer, therecan be advantageously employed a method similar to the aforementionedmethod for forming the i-type semiconductor layer. As the raw materialfor forming a deposited layer containing an element of the group IV ofthe periodic table, for obtaining a p-type semiconductor, a compoundcontaining an element of the group III of the periodic table is employedas a valence electron controlling agent. Such element of the group IIIcan be B, and specific examples of the compound containing B includeBF₃, B₂H₆, B₄H₁₀, B₅H₉, B₅H₁₁, B₆H₁₀, B(CH₃)₃, B(C₂H₅)₃, and B₆H₁₂.

Also for obtaining an n-type semiconductor, a compound containing anelement of the group V of the periodic table is employed as a valenceelectron controlling agent. Such element of the group V can be P or N,and specific examples of the compound containing such element includeN₂, NH₃, N₂H₅N₃, N₂H₄, NH₄N₃, PH₃, P(OCH₃)₃, P(OC₂H₅)₃, P(C₃H₇)₃,P(OC₄H₉)₃, P(CH₃)₃, P(C₂H₅)₃, P(C₃H₇)₃, P(C₄H₉)₃, P(SCN)₃ and P₂H₄.

In the following, there will be given a detailed description on thecontrol of an oxygen concentration and a carbon concentration in thevicinity of a p/n interface of the present invention.

In a photovoltaic device of a tandem cell formed by stacking a pluralityof unit devices each having a pn or pin structure, at an interface of ap-type layer and an n-type layer between the unit devices, a delicatechange or a structural strain in a matching or an impurity amount in theinterface may cause an increase in the serial resistance and thereby adeterioration in the I-V characteristics, thus significantly affectingthe performance of the photovoltaic device. Also such p/n interface maybe seriously damaged by plasma at an initial discharge of filmformation, thereby leading to a deterioration in the interfacialcharacteristics.

The present inventor, as a result of intensive investigations, has foundthat addition of a small amount of oxygen and/or carbon in the vicinityof the interface of the n-type layer and the p-type layer facilitatescontrol of the interface, thus leading to an improvement in thecharacteristics, and thus accomplishing the present invention.

In the p-type layer and the n-type layer, there are respectivelyemployed conventional impurity elements B and P as the valence electroncontrolling agents, but, additionally in the present invention, anoxygen atom concentration and/or a carbon atom concentration is socontrolled as to have a maximum peak in the vicinity of the p/ninterface between the unit devices.

The oxygen atom concentration can be controlled so as to have a peak inthe vicinity of the p/n interface by controlling an introduction amountof a gas containing oxygen as a constituting element. In such anoperation, the peak oxygen concentration is preferably controlled withina range from 1×10¹⁸ to 1×10²² atoms/cm³.

The carbon atom concentration can be controlled so as to have a peak inthe vicinity of the p/n interface by controlling an introduction amountof a gas containing carbon as a constituting element. In such anoperation, the peak carbon concentration is preferably controlled withina range from 1×10¹⁶ to 1×10²⁰ atoms/cm³.

In the present invention, oxygen and carbon may be added in any othermethod, and the present invention is not restricted by such method ofaddition.

In the present invention, the “vicinity of the p/n interface” means aregion within ±100 nm from the p/n interface, between the unit devicesto the p-type layer side and the n-type layer side (namely, a regionhaving a thickness of 200 nm in total). A peak concentration presentwithin a range of ±10 nm is more preferred, as the effect of the presentinvention of improving the interfacial characteristics is furtherenhanced.

A distribution of the concentration can be a symmetrical distribution,having a peak at the position of the p/n interface (±0) and symmetricalto both sides, or a distribution having a peak in the p-type layer or inthe n-type layer. A distribution having a peak of the concentration at alight entrance side of the interface is considered effective inimproving the optical characteristics, while a distribution having apeak in an opposite side to the light entrance side is consideredeffective for preventing diffusion of the impurity element, preventing adamage by plasma and improving a film adhesion.

As explained in the foregoing, a particularly large effect can beobtained by having a peak oxygen concentration in the vicinity of thep/n interface and controlling such peak oxygen concentration within arange from 1×10¹⁸ to 1×10²² atoms/cm³. Also a particularly large effectcan be obtained by having a peak carbon concentration in the vicinity ofthe p/n interface and controlling such peak carbon concentration withina range from 1×10¹⁶ to 1×10²⁰ atoms/cm³.

Thus, by controlling the oxygen concentration and/or the carbonconcentration under such conditions, it is possible to effectivelyprevent diffusion of conductive type determining impurities in thep-type semiconductor and the n-type semiconductor and to improve theinterfacial characteristics. Also, the p- or n-conductive typedetermining impurity, if present in an excessively large amount,deteriorates the photoconductivity, thereby increasing the serialresistance. Addition of oxygen and/or carbon widens a control latitudefor the amount of such impurity, and also improves the opticalcharacteristics.

Furthermore, addition of oxygen and/or carbon allows a highly resistantinterface to form, which is less susceptible to a damage by plasma. Italso relaxes a structural strain in the interface, thus improving thematching at the interface, and the adhesion and stability of the films.

If the peak oxygen concentration is less than 1×10¹⁸ atoms/cm³ and/or ifthe peak carbon concentration is less than 1×10¹⁶ atoms/cm³, the effectsof the present invention may not be exhibited sufficiently. On the otherhand, if the peak oxygen concentration exceeds 1×10²² atoms/cm³ and/orthe peak carbon concentration exceeds 1×10²⁰ atoms/cm³, the interfacialstructure may become unstable, and there may result detrimental effectson the electrical and optical characteristics.

In the following, there will be shown examples of the present invention,but the present invention is not limited by such examples.

EXAMPLE 1

A solar cell of the present invention having triple cells as three unitdevices was produced using a CVD film forming apparatus shown in FIG. 6.In FIG. 6, there are shown a reaction chamber 600, a substrate 101, ananode 602, a cathode 603, a substrate heater 604, a grounding terminal605, a matching box 606, a RF power source 607, an exhaust pipe 608, avacuum pump 609, a film-forming-gas introduction pipe 610, valves 620,630, 640, 650, 660, 670, 680, 622, 632, 642, 652, 662, 672 and 682, andmass flow controllers 621, 631, 641, 651, 661, 671 and 681.

At first, a stainless steel (SUS 304) substrate 101 of a size of 5×5 cmwith a surface mirror-polished to 0.05 μm Rmax was charged in anunrepresented sputtering apparatus. After the interior of the apparatuswas evacuated to 10⁻⁵ Pa or less, Ar gas was introduced and set to aninternal pressure of 0.6 Pa, and a sputtering was executed with an Agtarget by generating a DC plasma discharge with a power of 200 W,thereby depositing Ag in a thickness of about 500 nm.

Then a sputtering was conducted by changing the target to ZnO butemploying the same internal pressure and electric power, therebydepositing ZnO in a thickness of about 500 nm.

After the formation of a lower electrode 102 in the foregoing steps, thesubstrate 101 was taken out and was mounted on the cathode 603 in thereaction chamber 600, and the interior thereof was sufficientlyevacuated by the vacuum pump 609 to a vacuum level of 10⁻⁴ Pa measuredwith an unrepresented ion gauge.

Then the substrate 101 was heated to 300° C. by the substrate heater604. After the substrate temperature became constant, the valves 620 and622 were opened and the mass flow controller 621 was so regulated as tointroduce SiH₄ gas at a flow rate of 30 sccm, from an unrepresented SiH₄container into the reaction chamber 600 through the gas introductionpipe 610.

In the foregoing, the unit “sccm” indicates a unit of flow rate, wherein1 sccm=1 cm³/min (in a normal state), and the flow rate is hereinafterrepresented in the unit of sccm.

Similarly the valves 640 and 642 were opened and the mass flowcontroller 641 was so regulated as to introduce H₂ gas at a flow rate of300 sccm, and the valves 650 and 652 were opened and the mass flowcontroller 651 was so regulated as to introduce PH₃ gas diluted to 5%with H₂ gas at a flow rate of 10 sccm.

After the internal pressure of the reaction chamber 600 was regulated to200 Pa, an electric power of 10 W was supplied from the RF power source607 through the matching box 606, thereby generating a plasma dischargeand depositing an n-type amorphous silicon layer 103 in a thickness of40 nm.

After the termination of the gas supply, the interior of the reactionchamber 600 was again evacuated to a vacuum level of 10⁻⁴ Pa or lower,and the valves 620, 622, 630, 632, 640 and 642 were opened to introduceSiH₄ gas at a flow rate of 30 sccm, H₂ gas at a flow rate of 300 sccmand GeH₄ gas at a flow rate of 5.0 sccm into the reaction chamber 600.Then an electric power of 20 W was supplied from the RF power source607, thereby generating a plasma discharge and depositing an i-typeamorphous silicon germanium layer 104 in a thickness of about 180 nm.

Then, by setting the mass flow controllers at a flow rate of 0 sccm andclosing the valves 620, 622, 630, 632, 640 and 642, the flow rates ofSiH₄, H₂ and GeH₄ gases were instantaneously shut down to 0 sccm. Alsothe RF power was turned off to 0 W to terminate the plasma dischargeafter the termination of the gas supply, the interior of the reactionchamber 600 was evacuated to a vacuum level of 10⁻⁴ or less, and thevalves 620, 622, 640, 642, 660 and 662 were opened to introduce SiH₄ gasat a flow rate of 1 sccm, H₂ gas at a flow rate of 300 sccm and BF₃ gasdiluted to 5% with H₂ gas at a flow rate of 10 sccm into the reactionchamber 600.

Then an electric power of 20 W was supplied from the RF power source607, thereby generating a plasma discharge to deposit a p-type layer 105in a thickness of 10 nm, thereby obtaining a bottom layer 114. Thisp-type layer was confirmed to be constituted by microcrystals of a grainsize of 2 to 10 nm, by a sample prepared on a glass substrate under thesame conditions in a cross-sectional observation under a transmissionelectron microscope (TEM).

Then an n-type layer 106 was deposited in the same manner as in theformation of the aforementioned n-type 103, and an i-type layer 107 wasdeposited with a thickness of 100 nm, in the same manner as in theformation of the aforementioned i-type layer 104 except that the flowrate of GeH₄ gas was changed to 2.5 sccm.

Then a p-type layer 108 was deposited in the same manner as in theformation of the aforementioned p-type layer 105, but, in the course offilm deposition, the valves 670 and 672 were opened and O₂ gas wasintroduced under such control of the mass flow controller 671 that theoxygen concentration gradually increased toward an interface with ann-type layer 109 to be formed next, thereby completing a middle layer115.

Then, at the deposition of an n-type layer 109, O₂ gas was introduced byopening the valves 670 and 672 and regulating the mass flow controller671. The flow rate of O₂ gas was so controlled that the oxygenconcentration became continuous with that in the middle p-type layer108, and had a peak in the vicinity of the interface and graduallydecreased thereafter.

Then, SiH₄ gas at a flow rate of 30 sccm and H₂ gas at a flow rate of300 sccm were introduced, and an electric power of 20 W was applied todeposit an i-type amorphous silicon layer 110 in a thickness of 70 nm,and a p-type layer 111 was deposited to complete a top layer 116.

Then, after cooling, the substrate bearing the formed semiconductorlayers was taken out from the reaction chamber 600 and placed in anunrepresented resistance-heated evaporation apparatus. After theinterior of the apparatus was evacuated to 10⁻⁵ Pa or less, oxygen gaswas introduced and set to an internal pressure of 50 Pa, and an In—Snalloy was evaporated by resistance heating, thereby depositing atransparent conductive film (ITO film (indium tin oxide film)) servingas an upper electrode 112 and also having an antireflective function.

After the evaporation, the substrate was taken out, then separated intosub cells of a size of 1 cm×1 cm in an unrepresented dry etchingapparatus, and in another evaporation apparatus, an aluminum collectingelectrode 113 was formed thereon by electron beam evaporation. A solarcell thus obtained is indicated as Sample No. 1-1.

Also, Samples Nos. 1-2, 1-3, 1-4 and 1-5 were produced in the samemanner as explained in the foregoing, except that the O₂ gas flow ratewas changed in the middle p-type layer 108 and in the top n-type layer109.

FIG. 2 shows a distribution of oxygen concentration in the filmthickness direction of these samples, particularly in the vicinity ofthe interface between the top n-type layer and the middle p-type layer.A SIMS measurement was used for determining the oxygen concentration,and confirmed that the oxygen atom concentration had a peak value at aside of the top n-type layer in the vicinity of the n/p interface.

Also each sample was irradiated in a solar simulator with a light ofsolar spectrum of AM-1.5, with an intensity of 100 mW/cm², to obtain avoltage-current curve thereby determining an initial conversionefficiency of the solar cell. Obtained results are shown in FIG. 3.

FIG. 3 shows an initial conversion efficiency η as a function of a peakoxygen concentration in the top n-type layer in each sample, in theabscissa. The initial conversion efficiency η is represented in anormalized value, taking the initial conversion efficiency of the sample1-2 as 1.

A lower limit of the initial conversion efficiency, preferred for thepractical performance and the reliability of the solar cell, is definedas 0.95 in the above-described normalized value. It will be seen that aninitial conversion efficiency equal to or higher than such lower limitcan be obtained under the condition that a peak oxygen concentration inthe vicinity of the p/n interface is within a range from 1×10¹⁸ to1×10²² atoms/cm³.

EXAMPLE 2

The film-forming apparatus shown in FIG. 6 was used to produce a triplecell in the same manner as in Example 1, except that O₂ gas used forforming the middle p-type layer and the top n-type layer was changed toCH₄ gas. More specifically, CH₄ gas was supplied by opening the valves680 and 682 and regulating the mass flow controller 681, and SamplesNos. 2-1, 2-2, 2-3, 2-4 and 2-5 were produced by varying the flow rate.

FIG. 4 shows a distribution of carbon concentration in the filmthickness direction of these samples, particularly in the vicinity ofthe interface between the top n-type layer and the middle p-type layer.A SIMS measurement was used for determining the carbon concentration,and confirmed that the carbon atom concentration had a peak value at aside of the middle p-type layer in the vicinity of the p/n interface.

Also, each sample was subjected to a measurement of an initialconversion efficiency as in Example 1, and the results obtained areshown in FIG. 5.

FIG. 5 shows an initial conversion efficiency η as a function of a peakcarbon concentration in the middle p-type layer in each sample, in theabscissa. The initial conversion efficiency η is represented in anormalized value, taking the initial conversion efficiency of Sample No.2-2 as 1.

As in Example 1, a lower limit of the initial conversion efficiency,preferred for the practical performance and the reliability of the solarcell, is defined as 0.95 in the above-described normalized value. Itwill be seen that an initial conversion efficiency equal to or higherthan such lower limit can be obtained under the condition that a peakcarbon concentration in the vicinity of the p/n interface is within arange from 1×10¹⁶ to 1×10²⁰ atoms/cm³.

EXAMPLE 3

The film-forming apparatus shown in FIG. 6 was used to produce a triplecell in the same manner as in Example 1. In this example, however, O₂gas and CH₄ gas were simultaneously supplied for the middle p-type layerand the top n-type layer. More specifically, O₂ gas and CH₄ gas weresupplied by opening the valves 670, 672, 680 and 682 and regulating themass flow controllers 671 and 681, and samples were produced by varyingthe flow rate. In this operation, the samples were produced in such amanner that the peak oxygen concentration and the peak carbonconcentration were present at the side of the middle p-type layer in thevicinity of the p/n interface, and there was investigated therelationship between the peak oxygen concentration, the peak carbonconcentration and the initial conversion efficiency as in Examples 1 and2. As a result, there were obtained results similar to those in Examples1 and 2, and it was confirmed that optimum values for the peak oxygenconcentration and the peak carbon concentration in the vicinity of thep/n interface were the same as those in Examples 1 and 2.

EXAMPLE 4

The film-forming apparatus shown in FIG. 6 was used to produce a triplecell in the same manner as in Example 1. In the present example,however, samples were produced in such a manner that the peak oxygenconcentration was present at the side of the middle p-type layer in thevicinity of the interface between the middle p-type layer and the topn-type layer, and the relationship between the peak oxygen concentrationand the initial conversion efficiency with changing the oxygenconcentration was investigated as in Example 1. As a result, there wereobtained results similar to those in Example 1, and it was confirmedthat an optimum value for the peak oxygen concentration in the vicinityof the p/n interface was similar to that in Example 1.

As explained in the foregoing, preferred examples of the presentinvention provide the following effects.

Production of a photovoltaic device under the aforementioned conditionseffectively prevents diffusion of conductive type determining impuritiesin the p-type semiconductor and/or the n-type semiconductor, andimproves the interfacial characteristics. Also, even if the p- orn-conductive type determining impurity is present in an excessiveamount, though it deteriorates the photoconductive rate therebyincreasing the serial resistance, addition of oxygen and/or carbonwidens a control latitude for the amount of such impurity and alsoimproves the optical characteristics.

Furthermore, addition of oxygen and/or carbon forms a highly resistantinterface, which is less susceptible to a damage by plasma. It alsorelaxes a structural strain in the interface, thus improving thematching at the interface, and the adhesion and stability of the films.

Such improvement in the interfacial characteristics provides aphotovoltaic device of a high photoelectric conversion efficiency, andfurther a photovoltaic device showing an excellent interfacial matchingand a high structural stability (film adhesion).

1. A photovoltaic device comprising a plurality of unit devices stacked,each unit device comprising a silicon-based non-single-crystalsemiconductor material and having a pn or pin structure, wherein anoxygen atom concentration has a maximum peak in a region within 10 nmfrom a p/n interface between the plurality of unit devices.
 2. Aphotovoltaic device according to claim 1, wherein a concentration at thepeak of the oxygen atom concentration is at least 1×10¹⁸ atoms/cm³ andat most 1×10²² atoms/cm³.
 3. A photovoltaic device comprising aplurality of unit devices stacked, each unit device comprising asilicon-based non-single-crystal semiconductor material and having a pnor pin structure, wherein a carbon atom concentration has a maximum peakin a region within 10 nm from a p/n interface between the plurality ofunit devices.
 4. A photovoltaic device according to claim 3, wherein aconcentration at the peak of the carbon atom concentration is at least1×10¹⁶ atoms/cm³ and at most 1×10²⁰ atoms/cm³.
 5. A photovoltaic devicecomprising a plurality of unit devices stacked, each unit devicecomprising a silicon-based non-single-crystal semiconductor material andhaving a pn or pin structure, wherein an oxygen atom concentration and acarbon atom concentration have maximum peaks in a region within 10 nmfrom a p/n interface between the plurality of unit devices.
 6. Aphotovoltaic device according to claim 5, wherein a concentration at thepeak of the oxygen atom concentration is at least 1×10¹⁸ atoms/cm³ andat most 1×10²² atoms/cm³, and a concentration at the peak of the carbonatom concentration is at least 1×10¹⁶ atoms/cm³ and at most 1×10²⁰atoms/cm³.